This invention relates to an optical transmission device effective for improving the optical coupling efficiency between an optical element such as a light emitting element or a light receiving element (a photo detector) and an optical transmission path such as an optical fiber in optical devices requiring optical coupling between such an optical element and such an optical transmission path.
Demand has increased sharply in recent years for optical transmission circuits such as an optical transmission module (which will hereinafter be referred simply to as the "module"). The module consists of three fundamental constituents, i.e., an electric circuit, light emitting/receiving elements and an optical coupling system between the light emitting/receiving elements and an optical fiber. Among them, the electric circuit and the light emitting/receiving elements can be constituted either discretely or integrally by use of semiconductors, hence, the cost of production tends to decrease.
This is not the case with an optical coupling system. Aside from an optical fiber having a large diameter such as a plastic fiber, the optical coupling system becomes unavoidably complicated in construction if the optical fiber consist of ordinary quartz or multicomponent glass, and the module cost is greatly affected by said complicated comstruction.
The undesirable characteristics described above become more pronounced with high diffusiveness (incoherence) of a light emitting element such as a light emitting diode (hereinafter called an "LED"), with a smaller light receiving diameter of a light receiving element (a photo detector), with a smaller product of a numerical aperture (hereinafter called "NA") and an opening diameter (core diameter) of the optical fiber. However, if the construction of the optical coupling system is made simple at the sacrifice of the coupling efficiency, the loss margin between the transmission and reception sides decreases. To compensate for the loss, the light emission output or the reception sensitivity must be improved without decreasing the transmission loss of the optical fiber. However, this also involves predetermined limits from the aspects of practical application, performance and economy. Therefore, an optical coupling system having a simple construction and having high coupling efficiency between the optical element and the optical fiber, particularly between LED and the optical fiber, is earnestly required. Moreover, seal of the optical element and its easy detachability to and from the optical fiber are necessary.
The conventional methods of optical coupling can be broadly classified in principle into the following three methods.
The first method simply places the optical element and the optical fiber in such a manner as to oppose each other, and minimizes the distance between them. A so-called "Burrus" type lies at the extreme of this concept. More definitely, the chip surface of LED is etched near an active layer, and the end of an optical fiber is buried there into a resin.
In this case, the optical coupling efficiency can be improved close to the theoretical limit. However, this type is not at all practical because the take-out of the fiber is a so-called "pig tail" type which is difficult to handle as a module, and various difficulties occur in the production steps such as etching and hermetic sealing. Therefore, the most ordinary structure of the first method is such that an element is hermetically sealed in a package having a glass window, and a ferrule into which the optical fiber is packed is placed in such a fashion as to oppose the package outside from it. In this case, however, the distance between the optical element and the optical fiber is unavoidably more than the thickness of the glass window. In the case of a 0.2 mm.phi. LED and step index type optical fiber having an opening diameter of 0.2 mm.phi. and a numerical aperture of 0.5, for example, it is known experimentarily that the optical coupling efficiency falls off by about 1 dB when the distance between them increases by 0.1 mm within the range of distance of up to 1 mm. Since the glass window is generally at least 0.6 mm thick, the optical coupling efficiency drops by about 6 dB due to the thickness of the glass window. The drop of about t dB can be calculated to a fiber length of from 400 to 2,000 m in the case of a multi-component glass or quartz fiber, hence, this is indeed a great loss in terms of a transmission module. To sum up, this first method involves the problem of a low optical coupling efficiency, though it is easy to practice.
The second optical coupling method disposes a beam condensing means between the optical element and the optical fiber or at the end of the optical fiber in order to contract the output light from the optical element or the optical fiber to the other.
In order to condense the output light, lenses such as ordinary convex lenses, spherical lenses or rod lenses are used. Alternatively, the tip of the optical fiber is worked to a pointed ball. This method would be somewhat effective from the aspect of principle, but practical problems are yet left for a module to which the optical fiber is fitted and removed by use of a connector. Since the optical beam is contracted by a lens system, the accuracy of positioning between the optical element and the optical fiber drops as much in the perpendicular plane. On the other hand, the accuracy of positioning of the optical element to a stem or the like in a practical package is in the order of from dozens to 100 .mu.m. Accordingly, it is practically necessary to bring the fiber end surface out of focus of the beam condensing system spaced apart from a beam waist. Thus, this method can provide the improvement of the coupling efficiency of only at most about 1 dB, and this method uses an increased number of optical components and makes the construction more complicated. In other words, the cost becomes too high in comparision with the effect obtained by the method. As described above, practical methods of improving optical coupling efficiency have not yet been established.
The practical package structure of the conventional light transmission/reception module is fundamentally based upon such a structure in which LED, an LED driving integrated circuit chip, a photo diode (PD) and a reception integrated circuit chip are sealed in separate containers, and for this reason, it is not easy to miniaturize the module. Moreover, since the number of the necessary components is great, the cost of production of the module can not easily be reduced. To satisfy these requirements, a structure such as shown in FIG. 1, for example, has been proposed. In accordance with this structure, a light receiving element (a photo detector) 2 is connected to a transparent substrate 1 via a wiring layer 3, and is placed in such a manner as to directly face an optical fiber 4. Another conventional structure disposes a shading region 5 between the transparent substrates 1' in order to prevent the escaping light as shown in FIG. 2. Though this structure can improve considerably the optical coupling efficiency, the size can not be reduced sufficiently because there is an inherent limit to the reduction of distance between the end of the optical fiber and the surface of the light receiving element. When an optical fiber having a large numerical aperture and a large core diameter is used, the optical coupling efficiency can not be improved sufficiently. When an LED is used as the light emitting element, it is difficult to take sufficient quantity of light into the optical fiber because the light emission by the LED has high diffusibility.
As the third method, the article "Packaging hybrid circuit fiber optic transmitters and receivers" in Electronic Packaging and Production, Jan., 1980, pp. 135-141, discloses a method of improving the optical coupling efficiency between an active area and an optical fiber by interposing an optical, mosaic-like face plate (FP) consisting of a fiber aggregate between the active area consisting of LEDs or the like and the optical fiber. However, since this method merely interposes the FP, the improvement of the optical coupling efficiency is not yet sufficiently high.